config: enable VNC clipboard parameter in vga_fmt

config: enable VNC clipboard parameter in vga_fmt

add option to use the qemu vdagent implementation to enable the VNC
clipboard. When enabled with SPICE the spice-vdagent gets replaced
with the QEMU implementation.

This patch does not solve #1406, but does allow copy and paste with a
running X-session, when spice-vdagent is installed on the guest.

Signed-off-by: Markus Frank <m.frank@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>